
where \({\bf F}=K_1\left(\zeta \right)\hat{\bf R}+({\rm i}/\gamma ^2)K_0\left(\zeta \right)\hat{\bf z}\) , \(\zeta =(1-\beta \cos \theta )\,\omega R/v_0\) , and \(\beta =v_0/c\) . We use Eq. (REF ) to obtain the BR curves in Fig. 2 of the main text, as it gives reasonable results for the velocities under consideration (e.g., as compared to a virtual-quanta analysis [1]}).
